In a passive optical network (PON) a service provider implements what is known as an optical line termination (OLT), which is the root of a fiber distribution network. A single optical fiber extends from the OLT, and multiple passive optical splitters are used to split the fiber into multiple branches. The endpoints or clients of the distribution network are formed by optical network units (ONUs), which correspond to customers or end users.
Data from the service provider to clients is broadcast over the fiber distribution network, and each individual client filters the broadcast data to select the data intended for the client. Clients send upstream data to the service provider using a form of time-division multiplexing, in which each client is assigned a fraction of the available time in which to send data. Dynamic bandwidth allocation (DBA) is typically used for upstream data, so that upstream bandwidth is allocated based on the changing needs of the clients, the priorities of different types of data, and the service level agreements (SLAs) between the service provider and the customers.
A good DBA algorithm is essential for optimal network performance and utilization. Many types of data traffic are bursty and highly variable. By carefully managing upstream bandwidth allocation, a provider can oversubscribe a network while still providing guaranteed levels of service to individual customers.
In certain environments, such as gigabit-capable PON (GPON), there are two forms of DBA: non-status reporting (NSR) and status reporting (SR). Each ONU may have several transmission virtual pipes (referred to as transmission containers or TCONTs), each with its own priority or traffic class. In non-status reporting DBA, an OLT observes upstream traffic and when a given transmission container is not sending idle frames during its scheduled fixed allocations, the OLT may decide to increase the bandwidth allocation to that TCONT (depending on customer SLAs, TCONT types and other information), thereby allowing the TCONT to utilize a larger bandwidth for a burst of traffic. Once the burst has been transferred, the OLT will observe a large number of idle frames from the TCONT, and will reduce the allocation to the TCONT to fixed level. Non-status reporting DBA has the advantage that it imposes no requirements on the ONU. The disadvantage of non-status reporting DBA is that the OLT does not know how much excess data is waiting to be transmitted at each TCONT. Rather, the OLT allocates additional upstream bandwidth until a TCONT starts sending idle frames. Without knowing in advance the complete picture (how much data each TCONT has to transmit) it is impossible to reach optimized performance (consistent and low latency) and network utilization.
In status reporting DBA, the OLT repeatedly polls all or some of the TCONTS of some or all attached ONUs for their data backlogs. Each TCONT or virtual pipe has one or more physical queues with integrated scheduling mechanisms implemented by the ONU. An ONU reports on each TCONT separately to the OLT. Each report message contains a logarithmic measure of the data in all respective physical queues associated with given TCONT. Upon receiving reports from every relevant TCONT of the system, the OLT can calculate an optimized bandwidth map based on applicable SLAs as well as the current backlog of each transmission pipe.
Ethernet PON (EPON) uses a status reporting DBA mechanism similar to the GPON status reporting mechanism described above.